This invention relates in general to a thin film magnetodielectric for absorbing electromagnetic radiation and, more particularly, this invention relates to a thin film magnetodielectric which has effective electric susceptibility and effective magnetic susceptibility which are nearly equal for a broad band of electromagnetic wavelengths, whereby absorption with minimum reflection of electomagnetic radiation over a broad band of frequencies is effected.
In many applications, it is desirable to absorb or minimize reflection of electromagnetic radiation to prevent interference with electrical circuits or to prevent biological damage to humans or animals. Thus, electrical equipment, such as microwave ovens, plasma heating devices, and some therapeutic medical equipment, radiate high levels of electromagnetic radiation which may be detrimental to adjacent equipment and to biological life. Moreover, electromagnetic radiation transmissions, such as radio and television, microwave, satellite and the like, saturate the atmosphere and may interfere with the proper functioning of electrical equipment. It is, thus, desirable to provide a structure which prevents penetration of either internally or externally generated electromagnetic radiation. In defense applications, it is desirable to thwart enemy radar transmissions which detect planes, ships and land based equipment, so that the equipment can operate without discovery.
As disclosed in U.S. Pat. No. 2,951,246, issued Aug. 30, 1960, Inventors Halpern et al, one technique for controlling the absorption of electromagnetic radiation in order to minimize reflection of incident radiation, is to provide a coating of high loss dielectric material which has a thickness dependent on the frequency of the electromagnetic radiation to be absorbed. As disclosed, a reflective material, such as metal, is coated with a layer, comprising nonconducting dielectric material having embedded therein electrically conductive metallic flakes of a nonmagnetic material. The thickness of the dielectric layer is a function of the wavelength of the electromagnetic radiation to be absorbed. This patent also discloses the use of ferromagnetic particles or flakes which may be added to the nonmagnetic flakes in order to increase the index of absorption of the layer. The metallic flakes are applied in a parallel orientation on the surface to be covered. It is also disclosed in this patent, that, by altering the direction of application by 90xc2x0 in successive coatings a layer may be made which is isotropic in the plane thereof. The technique disclosed in this patent is disadvantageous because it is only effective in absorbing electromagnetic radiation of a narrow bandwidth. There is no disclosure in this patent of providing a layer which is capable of absorbing a broad band of electomagnetic radiation.
Magnetodielectric materials have been proposed for use in the manufacture of electrical equipment. Thus, U.S. Pat. No. 4,048,102, issued Sep. 13, 1977, Inventors Borzyak et al and U.S. Pat. No. 4,184,972, issued Jan. 20, 1980, Inventors Pevzner et al, disclose a magnetodielectrical material including a dielectric binder having iron filler. There is no disclosure in these patents that the iron based magnetodielectric may be used for the absorption of a broad band of electromagnetic radiation. A more pertinent patent is U.S. Pat. No. 3,540,047, issued Nov. 10, 1970, Inventors Walser et al. This patent discloses a thin film magnetodielectric material which absorbs a narrow band of electromagnetic radiation. The magnetodielectric includes a plurality of thin film metallic elements individually arranged in an orderly array and suspended in a dielectric media so that all of the elements in the array have a common uniaxial anisotropy axis. Layers of thin film elements are stacked alternately with layers of dielectric material. Although this patent discloses a thin film magnetodielectric material which is operable in a narrow waveband, there is no disclosure in this patent of providing such a material which is effective in absorbing or minimizing reflection of electromagnetic radiation over a broad band of frequencies.
According to the present invention, there is provided a thin film magnetodielectric which absorbs with minimal reflection electromagnetic radiation over a broad band of wavelengths. The thin film magnetodielectric has effective electric susceptibility and effective magnetic susceptibility that are substantially equal for a broad band of wavelengths. Thus, reflection of incident electromagnetic radiation is prevented over said broad band. According to a feature of the present invention, a thin film magnetodielectric includes at least first and second layers of thin film magnetic elements that are surrounded by and supported in a dielectric medium. Each element has two major axes, usually of different length, and a thickness which is substantially less than the major axes. We shall denote these two major axes as xe2x80x9clongxe2x80x9d and xe2x80x9cshortxe2x80x9d, recognizing that in certain atypical cases they will have the same length. Each layer included a plurality of elements arranged in an orderly array such that the long axes of the elements in a layer are parallel to each other. The long axes of one layer are oriented at 90xc2x0 to the long axes of the other layer.